Performance of the LHCb vertex detector alignment algorithm determined with beam test data

LHCb is the dedicated heavy flavour experiment at the Large Hadron Collider at CERN. The partially assembled silicon vertex locator (VELO) of the LHCb experiment has been tested in a beam test. The data from this beam test have been used to determine the performance of the VELO alignment algorithm. The relative alignment of the two silicon sensors in a module and the relative alignment of the modules has been extracted. This alignment is shown to be accurate at a level of approximately 2 μ m and 0.1 mrad for translations and rotations, respectively, in the plane of the sensors. A single hit precision at normal track incidence of about 10 μ m is obtained for the sensors. The alignment of the system is shown to be stable at better than the 10 μ m level under air to vacuum pressure changes and mechanical movements of the assembled system.